Not Applicable.
Not Applicable.
(1) Field of the Invention
This invention is concerned with safety razors of the kind in which a blade unit assembly is carried by a handle and includes at least one blade with a sharp cutting edge which is moved across the surface of the skin being shaved by means of the handle. A blade unit may be mounted detachably on a razor handle to enable the blade unit to be replaced by a fresh blade unit when the blade sharpness has diminished to an unsatisfactory level, or it may be fixedly attached to the handle with the intention that the entire razor be discarded when the blade or blades have become dulled. Detachable and replaceable blade units are commonly referred to as shaving cartridges. The present invention is particularly concerned with the blades of safety razor blade units, and relates specifically to elongate blades having cutting edges which extend along the blades.
(2) Description of Related Art
Safety razor blade units generally include, in addition to the at least one blade, a guard which defines a surface for contacting the skin ahead of the blades as the blade unit is moved across the skin in the performance of a shaving stroke, and a so-called cap defining a surface for contacting the skin behind the blade(s). It is well known that to achieve the best possible performance the or each blade must be orientated with respect to the cap and guard surfaces so that it is inclined to a plane which is tangential to those surfaces (the xe2x80x9ctangent planexe2x80x9d). By the blade being inclined it is meant that the mid-plane of the sheet material of the blade, which plane bisects the angle defined between the edge facets that form the cutting edge at the blade tip, is at an angle, sometimes called the xe2x80x9cblade tangent anglexe2x80x9d, to the tangent plane. A blade can be fixed in position so that the blade tangent angle does not vary to any significant extent when the razor is being used, but it is also known to mount blades movably within the frame so that their blade tangent angles may change as a consequence of blade movements in the performance of a shaving stroke. In either case it has been the prior practice to support the or each blade in safety razor blade units so that a substantially planar blade element which includes the cutting edge and a blade section extending back from the faceted edge for at least several millimeters, lies at an appropriate angle to the tangent plane. In one well known construction planar blade elements are mounted on bent blade supports which have a forwardly and upwardly inclined portion on which the blade elements are carried, and upright rear portions which are slidably guided. in the frame in which the blades are mounted. The need to set the blade tangent angles by correct positioning of the blades during blade unit assembly is inconvenient and has a tendency to complicate the construction of the blade units as well as the assembly procedures themselves. Furthermore, in modern safety razors it is common to have two or three blades with parallel edges, and having a plurality of such blades lying in respective planes inclined to the tangent plane can inhibit blade unit flexibility should that be desired.
There have been proposals for safety razors with blade elements having circular cutting edges which extend around holes in the elements. Examples of such safety razors are those described in U.S. Pat. Nos. 5,201,253 and 5,490,329. Unlike a straight-edged blade, the blade elements with circular cutting edges can not be inclined to the sharpened edge at a desired angle and position with respect to the skin during shaving. Therefore, these blade elements need to be made to ensure the required shape and inclination of the edges, such as by special grinding techniques as taught by U.S. Pat. No. 5,490,329, and/or the required shape can be imparted to the circular edges by deforming the edges after sharpening, as described in U.S. Pat. No. 5,201,253. These proposals have been addressed specifically to overcome problems inherent with blade elements with circular cutting edges, and they have no obvious application to blades with straight edges which do not present the same difficulties.
In U.S. Pat. No. 5,010,646 there is described a safety razor blade unit incorporating blades which are bent so that a cutting portion along which the sharpened edge extends is at an angle of about 112xc2x0 to a base or guide portion, both portions also being corrugated for stress relieving purposes. Blades bent in this way have substantially reduced flexibility and do not offer any solution to the above mention drawbacks of the hitherto known planar blades.
There is described in U.S. Pat. No. 3,938,250 a composite blade assembly for a safety razor, which composite blade assembly includes a planar blade and a supplementary blade member fastened to the underside of the planar blade, the supplementary blade member being bent to increase the span between the cutting edges respectively provided on the planar blade and the supplementary blade member. Also, the blade tangent angle of the supplementary blade member is reduced by this, blade member being bent. The bent blade member is specifically adapted to the production of a composite tandem blade and it does not offer any obvious solution to the drawbacks of the known arrangements which limit flexibility by requiring the blades to be set at certain inclinations to provide the desired blade tangent angles.
With a blade of this form a relatively small tip section can be bent to achieve the desired blade tangent angle with the major part of the blade being substantially planar and lying parallel to the tangent plane, thereby enabling enhanced flexibility, especially in blade units equipped with multiple blades.
The present invention has for its objective to eliminate, or at least substantially alleviate the limitations of the prior art blade arrangements. The invention is directed particularly to a blade for a safety razor blade unit, comprising an elongate blade element with upper and lower surfaces, a forward edge section having a sharp cutting edge extending therealong and defined by surface portions which taper towards the tip of the cutting edge, and a substantially planar blade section extending rearwardly from the edge section and over which the upper and lower surfaces are parallel, wherein in a transition region between the planar blade section and the forward edge section the lower surface has a convex curvature and the upper surface is non-convex and is preferably concave.
According to an aspect of the invention there is provided a blade of the form described above wherein, along at least most of the length of the blade, the blade cross-section has a minimum second moment of area not greater than 1.0xc3x9710xe2x88x924 mm4.
The second moment of area of the blade cross-section is dependent on the size and shape of the cross-section and is a measure of blade flexibility. Although flexibility is dependent on the Young""s modulus of the blade material, razor blade materials currently used have Young""s moduli close to one another so the second moment of area is a good indicator of blade flexibility. The second moment of area is measured with respect to an axis, and by the minimum second moment of area it is meant the moment of area determined with respect to that axis which results in the lowest value for that blade cross-section. It is preferable that the minimum second moment of area does not exceed 0.5xc3x9710xe2x88x924 mm4 and blades having a minimum second moment area less than 0.35xc3x9710xe2x88x924 mm4, and even less 0.20xc3x9710xe2x88x924 mm4, e.g. around 0.14xc3x97104 mm4 or lower are possible.
In accordance with another aspect the invention provides a blade of the form defined above wherein a plane bisecting the tip of the blade edge is inclined to the mid plane of the planar blade section at an angle in the range of 10xc2x0 to 35xc2x0. With the forward edge section set at such an inclination an effective blade tangent angle can be assured with the planar blade section lying substantially parallel to the tangent plane. The angle between the tip bisector and the mid plane of the planar blade section is preferably in the range of 15xc2x0 to 30xc2x0, and more especially 20xc2x0 to 25xc2x0.
A blade in accordance with a further aspect of the invention has the forward edge section extending back from the tip of the cutting edge by a distance not greater than 1 mm, and preferably not greater than 0.5 mm.
According to a preferred embodiment the forward edge section extends back from the tip by a distance extending not substantially beyond the tapering surface portions, and in particular by a distance several times less, than the distance through which the planar section extends rearwardly from the forward section.
In accordance with the different aspects of the invention mentioned above, the blade element conveniently comprises a metal strip having a bend at the junction between the forward edge section and the planar blade section.
The invention also resides, in accordance with a further aspect, in a safety razor blade unit including a guard surface, a cap surface, and at least one blade comprising an elongate blade element with upper and lower surfaces, a forward edge section having a sharp cutting edge extending therealong and defined by surface portions which taper towards the tip of the cutting edge, and a substantially planar blade section extending rearwardly from the forward edge section and over which the upper and lower surfaces are parallel, wherein in a transition region between the planar blade section and the forward edge section the lower surface has a convex curvature and the upper surface is concave, whereby a plane bisecting the tip of the blade edge lies at an angle to a tangent plane that is tangential to the guard and cap surfaces greater than the angle of the mid plane of the planar blade section to the tangent plane.
The mid plane of the or each blade is preferably substantially parallel to the tangent plane that is tangential to the guard and cap surfaces. With the upper blade surface concave at the transition region, the tip of the cutting edge lies above the plane of the upper surface at the planar blade section. A preferred blade element has the edge tip at a height of up to 0.25 mm, in particular in the range of 0.05 mm to 0.15 mm, above the plane of the upper surface.
With a blade as provided by the invention, and the blade tangent angle can be effectively set during the production of the blades themselves. Furthermore, as the major part of each blade can be planar, two or more blades can be disposed in an essentially planar array enabling production of a blade assembly with substantially greater flexibility than hitherto attainable. The benefits of the invention are most marked when an upwardly inclined blade portion, at forward edge section is confined to a small region close to the cutting edge.
A plurality of blades, each in accordance with the invention as explained above, may be conveniently assembled into a blade assembly which can form a sub-unit for assembly with other components in the manufacture of a safety razor blade unit, the blades being so interconnected that their cutting edges are substantially parallel, and the planar sections of the blades are substantially coplanar. The blades can be conveniently interconnected by transverse strips attached to the undersides of the planar sections of the blades. These interconnecting strips may also serve to secure the blade assembly to a support structure of the blade unit.
In a safety razor blade unit incorporating the blades of the present invention it is advantageous if the planar sections of the blades are coplanar with, or lie in a plane parallel to, the tangent plane which is tangential to cap and guard surfaces defined by the blade unit. Such an arrangement enables maximum advantage to be taken from the blade flexibility that the invention makes available.
The blade of the invention can consist of a blade element comprising a metal, e.g. steel, strip having a bend at the junction between the forward edge section and the planar section. The blade material can be conventional and the cutting edge can comprise coatings as known in the art, including hard coatings, such as boron nitride, amorphous diamond or diamond-like-carbon, which coatings are preferably deposited before the blade element is bent near the tip although it would be possible for one or more coatings to be applied subsequent to bending of the blade element.
In accordance with yet another aspect of the invention, there is provided a method of making a blade according to the invention as described above, the method comprising the steps of:
providing a substantially planar elongate blade element blank having a sharp cutting edge extending therealong and defined by surface portions which taper towards the tip of the edge; and
bending the blade element blank in a region not greater than 1 mm back from the tip of the cutting edge so that the tip of the cutting edge is displaced away from the initial plane of the blank.
During the bending operation the tip of the cutting edge is preferably displaced to a position above an upper surface plane of the blank, e.g. to a height of up to 0.25 mm, such as 0.05 to 0.15 mm above that plane.
In addition the invention resides in tooling for making blades according to the invention, the tooling comprising a first member having a flat pressing surface extended forwardly by an inclined ramp surface, and a second member having a flat pressing surface facing the flat pressing surface of the first member, the members being guided for movement of the flat surfaces towards and away from each other for pressing a blade element blank therebetween.
To inhibit unwanted movements of the blade element blank during pressing, a rear stop can be provided for abutment with the rear edge of the blade element blank and/or the ramp surface can be limited by a forward stop disposed for engagement by the front edge of the blade element blank during the pressing thereof.
The blades of this invention are of particular benefit when used in a safety razor blade unit having a resiliently compliant blade support structure, such a blade unit being described and claimed in our copending British Patent Application 0025336.9 and an International patent application claiming priority therefrom, and the contents of which are incorporated herein by reference.